1. Field of the Invention
The present invention relates to particulate materials that are useful for fuel reformers, such as catalyst materials for hydrogen production from carbon-based fuels and absorbent materials for the removal of acid gases such as CO2 and H2S from gas streams. The particulate materials can be produced by spray processing of precursors to form a powder batch. The present invention is also directed to fuel reformers incorporating the particulate materials and methods for using the materials. The present invention is also directed to the combination of a highly reversible, high-capacity CO2 absorbent with steam reforming and/or water gas shift catalysts to achieve single step conversion of hydrocarbon fuels to hydrogen with a high conversion efficiency.
2. Description of the Related Art
Hydrogen (H2) is an important material in the chemical, petroleum and energy industries. In the chemical and petroleum industries, hydrogen is used for the manufacture of ammonia (NH4) and methanol (CH3OH), and is used in a variety of petroleum hydrotreating processes. A growing demand for hydrogen is forecast in the future, particularly for petroleum refining of heavy, high-sulfur crude oil. Hydrogen is also an environmentally clean energy source for the generation of electric power and space heating, and a substantial increase in hydrogen demand is expected in the near future.
Steam reforming, including steam-methane reforming (SMR), partial oxidation (POX) and autothermal reforming (ATR) are the major processes for hydrogen production from fossil-based fuels such as natural gas, and are expected to remain processes of choice for the next several decades. These fuel-processing technologies involve multiple steps and severe operating conditions. For example, SMR involves the endothermic reaction of methane (e.g., natural gas) with water to form H2 and carbon monoxide (CO). The primary reformer operates at a temperature of approximately 800° C. to 850° C. and about 20 atm of pressure, and large quantities of supplemental fuel must be burned to supply the energy necessary to maintain the reformer temperature. Further, the reforming step is followed by at least one water gas shift (WGS) reactor to increase the H2 content and reduce the CO content. This is followed by CO cleanup using selective oxidation, hydrogen membrane separation, pressure swing absorption or methanation. The reactions that occur during SMR of methane (CH4), the major component of natural gas, are illustrated by Equations 1 to 4:
ReformingCH4 + H2O → 3H2 + CO(1)ShiftCO + H2O → H2 + CO2(2)CleanupCO + O2 → CO2(3)or CO + 3H2 → CH4 + H2O(4)
Another commercially available method for the production of hydrogen from hydrocarbons is partial oxidation (POX). According to this method, CH4 or a similar hydrocarbon feed stock is oxidized to produce CO and H2 in accordance with the reaction illustrated by Equation 5:CH4+½O2→2H2+CO  (5)This reaction is exothermic and therefore no heat is required. A typical POX system includes a POX reactor, followed by a shift reactor and a H2 purification step. A POX reactor is more compact than a steam reformer since no additional heat must be added. The efficiency of the POX reactor is relatively high, however, POX systems are typically less energy efficient than SMR because of the utilization of higher temperatures and the problem of heat recovery.
Auto thermal reforming (ATR) combines some of the features of SMR and POX. In ATR, a hydrocarbon feed such as CH4 is reacted with both steam and air to produce an H2-rich gas. Both the SMR and POX reactions take place (Equations 1 and 5). With the correct mixture of input fuel, air and steam, the POX reaction supplies all the heat needed to drive the catalytic SMR reaction. However, as with SMR and POX systems, a WGS reactor and a H2 purification stage are required to remove carbon oxides.
Fuel cells provide electricity through chemical oxidation-reduction reactions and have tremendous advantages over other types of power generation devices in terms of energy efficiency and environmental compatibility. For low temperature applications, the most promising type of fuel cell is the proton exchange membrane (PEM) fuel cell, which employs H2 as a fuel in the anode and O2 as an oxidant in the cathode. However, the cost of constructing the distribution infrastructure to safely transport pure H2 gas over large distances presents an economic barrier to the exploitation of fuel cells, particularly for the transportation sector. Therefore, distributed production by smaller reforming systems that convert hydrocarbons to H2 gas is a more viable option for the near future. However, conventional fuel-processing technologies for H2 production from hydrocarbons are unsatisfactory for providing H2 to PEM fuel cells due to low reforming efficiencies resulting from the multiple steps that are required and the severe operating conditions that are required, as is discussed above. Further, the reformate typically has a low H2 content (45 to 50 mol. % on a dry basis) and a high CO and CO2 content. The reformate can also include other gases, such as N2, depending on the reforming method.
The low H2 content in the reformate reduces the fuel cell performance and requires that greater amounts of expensive Pt and PtRu catalysts and membrane materials be utilized for reasonable system efficiency as compared to fuel cells operating on pure H2. The high levels of CO2 in the reformate also cause two additional problems. The CO2 converts to CO in the PEM stack due to the reverse water gas shift reaction (reverse of Equation 2) and the CO can poison the catalyst. Also, the acidic nature of CO2 and water solutions promotes a number of reactions that can reduce the useful lifetime of the PEM stacks by greater than 50%.
The deficiencies of conventional fuel reforming processes can be overcome to a certain extent by following the WGS step with amine scrubbing, hydrogen membrane separation and/or pressure swing adsorption (PSA). With amine scrubbing it is often necessary to further reduce the concentration of carbon oxides to trace levels by methanation. PSA requires operation at a significant pressure, which lowers system efficiency and produces a tail gas containing 25% to 30% of the H2 produced during column blowdown and purge. While the energy content of the tail gas can be recovered and used in the reforming process it is often the case that the energy content of the combined fuel cell anode tail gas and the purification tail gas is greater than the energy required by the reforming process.
A variety of approaches have been explored to develop a fuel processing technology that uses simple chemical processes, has low energy consumption and generates high purity H2. These include the application of reaction-separation membranes and the application of absorption materials. One promising approach is absorption enhanced reforming (AER). AER combines a SMR catalyst and a CO2 absorbent (e.g., CaO) in a single reactor so that reforming, shift, and CO2 absorption occur simultaneously.
CO2 absorptionCaO + CO2 → CaCO3(6)OverallCaO + CH4 + 2H2O → 4H2 + CaCO3(7)
Many potential benefits over conventional reforming have been demonstrated using AER. These include: (i) reforming at a significantly lower temperature (about 600° C.), while achieving an increased conversion of CH4 to H2; (ii) lower capital cost as compared to conventional SMR; (iii) producing H2 at feed gas pressure (200 to 400 psig) and at relatively high purity (>95%) directly from the reactor; (iv) reducing or even eliminating downstream purification steps; (v) minimizing side reactions and increasing catalyst lifetime; (vi) reducing the excess steam used in conventional reforming, particularly when treating heavy fuels; and (vii) effective fixing of CO2.
It should be noted that various terminology has been used in the literature to describe the reaction of CO2 and a solid material such as CaO. Among the terms used are adsorption, absorption, sorption and fixing of CO2. In general, none of these terms precisely describes this complex process, which starts with adsorption of the CO2 onto the surface of a solid, followed by a chemical conversion of the solid and expansion of this process into the bulk of the solid. Therefore, the terms adsorption, absorption and fixing (to describe the process), and adsorbent and absorbent (to describe the solid material) are used interchangeably within the present specification.
The use of AER for H2 production for use in a fuel cell has been disclosed in U.S. Patent Application Publication No. 20020155329 by Stevens. More recently, the benefits of this approach for hydrogen production from solid fuels such as biomass and coal have also been demonstrated by S. Lin et al. (Fuel 2002, 81, 2079).
There are a variety of CO2 absorption materials available for AER. Reaction-based CO2 absorption materials such as CaO-based absorbents are preferred because these types of materials typically have much higher equilibrium capacities than other absorbents. For example, under ideal conditions methylethanolamine captures 6 g/100 g (grams of CO2 per 100 gram of material), silica gel absorbs 1.32 g/100 g, and activated carbon absorbs 8.8 g/100 g. Materials used for PSA such as K2CO3/Hydrotalcite can only remove a small portion of CO2, about 1.98 g/100 g. In contrast, CaO can capture 78.57 g/100 g. Even assuming only a 50 wt. % capacity over repeated cycles, the value of 39.3 g/100 g for AER is 5 to 10 times higher than the above absorbents.
The conversion of hydrocarbons in the presence of steam and a CO2 absorbent can be traced back to as early as 1868. Recently some results for hydrogen production using this concept have been reported by: D. P. Harrison et al, Chemical Engineering Science 1999, 54, 3543; V. K. Ravi et al., Proceeding of the 2002 U.S. DOE Hydrogen Program Review NREL/CP610-32405; M. Spectht et al., Hydrogen Energy Progress XIII, Z. Q. Mao, T. N. Veriroglu (Eds.), 2000, 1203; and J. R. Hufton et al., Proceedings of the 1995 U.S DOE Hydrogen Program Review, 1995, 1, 815. The CO2 absorbents typically used for AER have poor reactivity, low CO2 capacity, and poor recyclability. The key to successfully commercialize AER methods is to develop an absorbent with high activity and capacity, and particularly with high recyclability to maintain sufficient activity and capacity over numerous carbonation and decarbonation cycles.
The absorbent materials have other applications in addition to AER. The increasing use of fossil fuels to meet energy needs has led to higher CO2 emissions into the atmosphere and CO2 emissions from direct combustion of fossil fuels account for one-half of the greenhouse effect that causes global warming. It is therefore necessary to develop cost-effective CO2 management schemes to curb CO2 emissions. Many CO2 management schemes consist of three parts: separation, transportation, and sequestration. The capture of CO2 accounts for about 75% of the total cost of CO2 management, and imposes a severe energy requirement on fossil fuel-based power plants, reducing their net electricity output by as much as 37%. The costs associated with current CO2 separation technologies necessitate the development of economical alternative.
Integrated gasification combined cycle (IGCC) power generation systems hold great promise for producing electric power efficiently and economically. Instead of direct combustion, coal is gasified by means of enriched air, O2, and/or H2O to obtain a concentrated coal gas. After cleanup, the coal gas is used to fire a gas turbine. The hot exhaust from the gas turbine is passed to a boiler to generate steam, which is then used to drive a steam turbine.
Coal includes sulfur as an impurity, which upon gasification typically enters the coal gas stream as hydrogen sulfide (H2S). It has been common practice to cool the gas below 77° C. to remove H2S by wet scrubbing. However, this practice reduces the overall efficiency of the power generation plant significantly. Hot gas cleanup methods capable of operating at temperatures close to the gasifier outlet temperature, which may range from 725° C. to 1325° C., would increase the overall energy conversion efficiency. While various absorbents have been proposed for desulfurizing hot coal gas, few are effective at such high temperatures.
It is known that zinc-based materials such as zinc oxide (ZnO) are highly effective for removing H2S, but their use is limited to temperatures below 650° C. At higher temperatures, calcium-based absorbents are promising because the reaction of CaO with H2S is both thermodynamically and kinetically favorable. The reactions are illustrated by Equations 8 to 11.
AbsorptionCaO + H2S → CaS + H2O (8)OxidationCaS + 2O2 → CaSO4 (9)ReductionCaSO4 + CO → CaO + CO2 + SO2(10)CaSO4 + H2 → CaO + H2O + SO2(11)
It has been proven experimentally and thermodynamically by van der Ham et al. (Ind. Eng. Chem. Res., 1996, 35, 1487) that the H2S content of gas produced by an air- and steam-blown coal gasifier operating under typical conditions can be reduced to 20 ppmv by employing CaO in highly reducing conditions at a temperature higher than 800° C. Regeneration can be accomplished at a temperature above the absorption temperature by employing a cyclic oxidation and reduction process, as is disclosed by S. B. Jagtap et al., Energy Fuels, 1996, 10, 821 and in U.S. Pat. No. 6,083,862 by Wheelock.
Natural CaO-based absorbents such as limestone and dolomite are plentiful and inexpensive, but they are soft and friable and do not stand up well to handling and recycle use. To improve the recyclability, some work has been focused on the pelletizing of limestone by using different binders. See, for example, U.S. Pat. No. 4,316,813 by Voss et al. Some work also focused on the modification of natural materials such as dolomite to tailor the physicochemical properties of the material. The synthesis of a CaO-based absorbent through boiling of CaO into Ca(OH)2 or the carbonation of calcium salt solution such as calcium nitrate or Ca(OH)2 into calcium carbonate, then decomposition of the carbonate into CaO has been disclosed by L. S. Fan et al., Ind. Eng. Chem. Res., 1999, 38, 2283. Others have disclosed the preparation of CaO-based materials by aerogel methods.
The foregoing methods generally result in limited control over the composition and microstructure of the powders. The morphology and surface properties such as surface area, pore volume and pore size are among the characteristics that have a critical impact on the performance of the absorbent. This is due to the nature of the reactions that occur. First, carbonation takes place on the external and internal surfaces of CaO-based absorbent, which forms a carbonate layer. Then, the chemical reaction advances with the diffusion of CO2 through the carbonate layer into the unreacted core CaO active sites. Therefore, higher reactivity and faster kinetics can be expected for small particle size CaO due to the higher surface to bulk ratio of the absorbent species. A more porous structure will also lead to higher reactivity and recyclability, and a lower decarbonation temperature due to the easier CO2 diffusion into or out of the outer carbonate layer.
It would be advantageous to provide a method for producing absorbent powders that would enable control over the powder characteristics such as particle size, surface area and pore structure as well as the versatility to accommodate compositions which are either difficult or impossible to produce using existing production methods. It would be particularly advantageous if such powders could be produced in large quantities on a substantially continuous basis. Further value can be derived from these powders if they can be incorporated into structures that can be integrated into reactor beds that enable a suitable combination of high space velocity and high absorption capacity while retaining their performance characteristics. Such structures include coatings, such as wash coatings on highly porous monoliths, pellets that have pore structures that retain the performance of the powders and also coatings or impregnation of the powder particles into other structures such as metals, metal oxides, textiles or cloths, which may provide beneficial heat transfer characteristics.